It is necessary that a size of a switch mutually connecting logic cells has to be reduced and the ON resistance thereof has to be reduced in order to diversify a function of the programmable-logic and to promote mounting on an electric apparatus. A non-volatile switching element, which switches from the OFF state to the ON state, has been developed in which a metal is deposited in an ion conduction layer conducting a metal ion to form a metal bridge in the ion conduction layer by using an electrochemical reaction. It is known that the non-volatile switching element has smaller size and lower ON resistance compared with a conventional semiconductor switch. As the non-volatile switching element, “two-terminal switch (FIG. 1A)” disclosed in Patent Literature 1 and “three-terminal switch” disclosed in Patent Literature 2 are known. the “two-terminal switch” shown in FIG. 1A has a structure in which an ion conduction layer is sandwiched between a first electrode supplying a metal ion and a second electrode supplying no metal ion, in a step of changing the switching element from the “OFF” state to the “ON” state. In the step of changing the switching element from the “OFF” state to the “ON” state, the second electrode is grounded and a positive voltage is applied to the first electrode. In the first electrode side, a metal is ionized and a generated metal ion is introduced into the ion conduction layer. In the second electrode side, the metal ion is reduced and the metal deposits. Since the deposited metal forms a metal bridge extending from the second electrode to the first electrode in the ion conduction layer, switching from the “OFF” state to the “ON” state is achieved. In a step of changing the switching element from the “ON” state to the “OFF” state, the second electrode is grounded and a negative voltage is applied to the first electrode. At this time the deposited metal is re-ionized, re-deposition of the metal progresses due to reduction of the metal ion, consequently the metal bridge disappears and switching from the “ON” state to the “OFF” state is achieved.
Since the “two-terminal switch” has a simple structure, a manufacturing process is simple and the “two-terminal switch” having an element size of nanometer order can be manufactured. Since the “three-terminal switch” includes, as exemplified in FIG. 3 (FIG. 1B) in Patent Literature 3, a structure in which two second electrodes of the “two-terminal switch” are integrated, high reliability is acquired.
A porous polymer including silicon, oxygen, and carbon, as primary components is desirable as the ion conduction layer. The porous polymer ion conduction layer can keep an “insulation breakdown voltage” high even though the metal bridge is formed, and therefore excels in operation reliability (Patent Literature 3).
It is necessary to decrease “element size” and simplify manufacturing steps in response to densification of wiring in order to mount (apply) the non-volatile switching element as a programmable-logic wiring changeover switch. The most advanced semiconductor device mainly employs copper as a wiring material which is used for forming of multi-layered wiring. Development of a technique of effectively forming a non-volatile switching element such as variable resistance element in a copper wiring of a multi-layered structure is required. Non-Patent Literature 1 discloses a technology of integrating a switching element using an electrochemical reaction into a semiconductor device. Non-Patent Literature 1 describes a structure in which a copper wiring on a semiconductor substrate is used as a first electrode of the switching element when the first electrode of the switching element is manufactured by using copper. If the structure is used, a step in which the first electrode is newly formed in addition to the copper wiring can be omitted. A mask for a “patterning step” used for forming the first electrode is not required, and two photo masks (PR) used in a step of forming the “ion conduction layer” and a step of forming the “second electrode” are only added in order to manufacture a variable resistance element having a structure of the “two-terminal switch”.
When the copper wiring on the semiconductor substrate is used as the first electrode of the switching element, if the “porous polymer ion conduction layer” made from a porous polymer including silicon, oxygen, and carbon as primary components is directly formed on the copper wiring, a surface of the copper wiring is oxidized. After a thin metal film which works as an oxidation sacrifice layer is formed on the copper wiring surface in order to prevent oxidation of the surface of the copper wiring, the “porous polymer ion conduction layer” is formed. The thin metal film is oxidized by oxygen during the step of forming the “porous polymer ion conduction layer” and changed into a “thin film of metal oxide with ion conductivity”. As exemplified in FIG. 4 (FIG. 1C) in Patent Literature 3, the “thin film of metal oxide” generated by oxidation of the “thin metal film which works as the oxidation sacrifice layer” composes the ion conduction layer with the “porous polymer ion conduction layer” which is formed thereon.
When the first electrode of the switching element is manufactured by using copper, the second electrode, which does not supply a metal ion during changeover of the switching element from the “OFF” state to the “ON” state, is made from platinum or gold which is difficult to be oxidized or ruthenium which keeps conductivity even though it is oxidized. In Non-Patent Literature 1, the second electrode is formed by using ruthenium which is suitable for manufacturing.